sounds like the screw has screwed it self into something in the third layer and has now broken it off so it is rotating as you turn the screw. at this poing it would be almost impossible to simply unscrew it without a lot of pressure(i dont like doing that because bad things can happen.) . if there is room for the screw to fall through and it is spinning freely in the sheet metal you could try cutting the screw head off the screw and let it fall through. then tap out new threads. and insert a new screw or bolt.